In order to energise a magnetron, so that it will oscillate and generate a high frequency output pulse, it is necessary to apply across its anode and cathode a voltage pulse normally exceeding 1 kilovolt, which may be referred to as a firing pulse, and the magnetron then oscillates for a period approximately equal to the duration of the firing pulse. The power of the firing pulse normally will exceed 1 kilowatt. Because of the relatively high voltage and power involved, there have been special problems associated with developing circuitry capable of switching a firing pulse onto a magnetron. The circuits employed have generally been referred to in the art as modulators. One of the desirable characteristics of a modulator is that is should be capable of generating firing pulses of different lengths, so as to enable generation of magnetron output pulses of corresponding different lengths.
One type of modulator which has been employed is referred to as the line type modulator. This type utilises a delay line to define the firing pulse length, and in order to change this length more than one delay line is provided, these having different delay values, and relay switches are employed to switch between one line and another. The energy passing through the circuit at the point of switching is relatively high so that the problems normally associated with mechanical switching at relatively high energy levels occur. The output pulse from the delay line is applied to a pulse transformer which steps up its voltage for application across the magnetron. The output impedance of a line type modulator is roughly equivalent to that of the pulse transformer, with the result that only about half of the voltage which might nominally be expected can actually be made available across the transformer primary winding. Consequently, the turns ratio of the pulse transformer has to be increased to achieve the desired output voltage in the firing pulse. The higher the turns ratio, the narrower the range of different pulse lengths that may be satisfactorily transmitted through the pulse transformer.
Another prior type of modulator is known as the hard valve type modulator. This employs a high voltage vacuum valve which switches a voltage of several kilovolts directly across the magnetron. The hard valve has the advantage that it can be switched from conduction to non-conduction by means of a relatively low control voltage, and the output pulse length can be varied by varying the length of the input control pulses at this relatively low voltage level. However, the valve is large, requires typically between 10 and 20 watts of power for heating of its cathode, and requires a constant supply voltage of several kilovolts to be provided for switching on to the magnetron.